Download atomic mass

Warm Up
2/10
• What do you know about atoms and the
periodic table?
• What do you think an atom looks like?
Atomic Structure
• ATOM: the smallest particle that has the
properties of an element.
• From the early Greek concept of the atom to
the modern atomic theory, scientists have
built on and modified existing models of the
atom.
• Atoms are composed of a positively charged nucleus
surrounded by an electron cloud.
– Nucleus (99% of atom’s mass): uncharged
neutrons and positively charged protons.
– Electron cloud: negatively charged electrons in
constant motion creating a “cloud” like a fan.
DEMOCRITUS
• In 400 B.C., this Greek
philosopher suggested that
the universe was made of
indivisible units.
• “Atom” (atomos)– Greek
word meaning “unable to
be divided”
JOHN DALTON
In 1808, this English teacher
proposed his own atomic theory
that became widely accepted.
Dalton’s Atomic Theory:
–
–
–
–
All elements are made of tiny atoms.
Atoms cannot be subdivided.
Atoms of the same element are exactly alike.
Atoms of different elements can join to form
molecules.
Do these ideas still hold true today?
• As it turns out, the atom can be divided into
subatomic particles.
• Thomson and Millikan are given credit for the first
discoveries relating to electrons.
http://www.shs.d211.org/science/faculty/hlg/history%20of%20atom%20sanjay%20karan/millikanpage.htm
•Rutherford discovered the positively charged nucleus
(GOLD FOIL EXPERIMENT)
NIELS BOHR
• In 1913, this Danish scientist suggested
that electrons “orbit” the nucleus.
• In Bohr’s model, electrons are placed in
different energy levels based on their
distance from the nucleus.
1869: created first periodic table of elements.
Arranged elements in order of increasing atomic
mass.
One of Rutherford’s students.
1914: Arranged the elements in order of increasing
atomic number
(responsible for TODAY’S periodic table).
Electrons are like books in a book case.
They can only exist on certain levels.
Only certain orbits are allowed
The Bohr Atom was a “Solar
System” model.
MODERN ATOMIC
MODEL
• By 1925, Bohr’s model of the atom no longer
explained all observations. Bohr was correct about
energy levels, but wrong about electron movement.
• Electrons occupy the lowest energy levels available.
• Energy increases as distance from the nucleus
increases.
• Electrons move in patterns of “wave functions”
around the nucleus.
• It is impossible to know an electrons velocity and
location at any moment in time (Schrödinger).
Schrödinger
This is very
difficult stuff!
In 1926 Schrodinger proposes
an equation that gives the
probability of finding an electron
Don’t worry, this
won’t be on the
at any place in the atom.
test.
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 V  i
2
2m x
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2
2
ORBITALS
• ORBITAL: the regions in an atom where there is a high
probability of finding electrons.
• s is the lowest energy orbital, and p is slightly higher
“Spherical”
“Dumbbell”
shaped
• d and f are the next two orbitals. They
occupy even higher energy levels and
take on more complex shapes than s &
p
REMEMBER…
 We
always fill in this order: s, p, d, f
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VALENCE ELECTRONS
• Electrons in the outermost energy level are called
VALENCE ELECTRONS.
• Valence electrons determine how an atom will act in
a chemical reaction.
• Atoms with equal numbers of valence electrons have
similar properties.
• We will learn how to determine the # of valence
electrons in an atom later in this unit.
Review Questions
What part of the atom is much smaller than the
atom, yet contains most of the mass?
a) the nucleus
b) the electron cloud
Not drawn to scale
While its diameter is very small compared to that
of the entire atom, 99% of the mass of an atom
comes from the protons and neutrons in the
nucleus.
Which statement is true according to Dalton’s
atomic theory?
a)
b)
c)
d)
Atoms of different elements join to form larger atoms.
Atoms can be subdivided into smaller particles.
Atoms of the same element differ in electric charge.
Atoms of the same element are exactly alike.
This statement was part of Dalton’s atomic theory.
While this statement was holds true if we remove
the word “exactly”, we have since found that
atoms of an element often differ in number of
neutrons (isotopes).
According to Bohr’s model of the atom,
electrons behave like
a)
b)
c)
d)
planets rotating on their axes.
planets orbiting the sun.
light energy in a vacuum.
waves vibrating on a string.
While Bohr’s model of energy levels of
Schrödinger
electrons around an atom remains supported,
his model of electron movement has been
revised. Schrödinger and other scientists have
since found that electrons travel in wave-like
patterns around the nucleus.
Organization of the Periodic Table
• PERIODICITY: regular variations (or
patterns) of properties with increasing
atomic number. Both chemical and
physical properties vary in a periodic
(repeating) pattern.
• PERIOD: horizontal row of elements on
P.T.
•GROUP (FAMILY):
vertical column of elements on P.T.
Periodic Key
# of protons = Z
# of electrons = # of protons (in a neutral atom)
6
# of neutrons = A-Z
Atomic number (Z)
C
Element’s symbol
Carbon
Element’s name
12.011
Atomic mass (A)
Determining # P+, N, and E- from
chemical symbols:
Mass #
• Example 1: atom
# protons = 6
#electrons= 6
#neutrons= 14-6 = 8
No net
charge
14
6
C
Atomic #
• Example 2: ion
# protons = 7
#electrons= 10
#neutrons= 15-7 = 8
15
7
N3-
Net
charge
of -3
ISOTOPES
• Isotopes are atoms that have the same # of
protons, but a different # of neutrons.
• Example:
Carbon-12 vs. Carbon-14
12C
Mass # = 12; Atomic # = 6 (6P, 6E, 6N)
14C
Mass # = 14; Atomic # = 6 (6P, 6E, 8N)
IONS
• Ionization: the process of adding or removing
electrons from an atom or group of atoms.
• An ion has a net electric charge.
• Cation: ion with a positive charge.
Ex: Na+
• Anion: ion with a negative charge.
Ex: O2-
Valence electrons &
electron dot diagrams
• Review: The valence electrons are the
outermost electrons in an atom. These are
the electrons that are involved when there is
a chemical reaction.
• Looking at a periodic table, you can quickly
determine how many valence electrons an
atom has by what column it is in.
# of Valence electrons
2
1
2
3
4
5
6 7 8
ELECTRON DOT DIAGRAMS:
visual representations of elements and their valence electrons
Standard form:
Order of electron/dot
placement
36
4
7
R
58
1
2
element symbol
Example:
Oxygen
O
6 valence
electrons
How many dots are in an electron dot
diagram of chlorine?
a)
b)
c)
d)
1
7
17
35
Chlorine and all of the other
halogens (column 17) have
seven valence electrons.
Cl
Review Questions…
How many protons, electrons and
neutrons are in one atom of oxygen-17?
a)
b)
c)
d)
17 p, 17 e, 17 n
17 p, 17 e, 1 n
8 p, 8 e, 8 n
8 p, 8 e, 9 n
While most oxygen atoms have a mass of 16 g/mol, oxygen-17 is
an isotope with a mass of 17 g/mol.
The number of protons in an element is the same for every atom
of that element.
# electrons = # protons if element has no charge.
The periodic law states that when elements are
arranged in order of increasing __________
_________, similarities in their properties occur in
a regular pattern.
a) Atomic mass
b) Atomic number
c) Atomic radius
Moseley created the modern periodic table
when he determined that elements should
be placed in order of increasing atomic
number (# of protons). It’s a shame that
WWI took the life of this brilliant scientist
when he was so young.
Carbon-12 and carbon-14 are
a)
b)
c)
d)
isomers
isotopes
radioactive elements
different elements
Isotopes are atoms of the same element with
different numbers of neutrons, and therefore
different atomic masses.
While carbon-14 is used in radioactive dating,
carbon-12 has a more stable nucleus and therefore
is not used in this capacity.
How many protons, electrons and
neutrons are in O2-?
a)
b)
c)
d)
10 p, 8 e, 8 n
8 p, 8 e, 8 n
8 p, 10 e, 8 n
8 p, 6 e, 8 n
Every atom or ion of oxygen has 8 protons.
The net charge of -2 indicates that the ion has two
more negative charges (electrons) than positive charges
(protons)
All atoms and ions of standard oxygen-16 have 8
neutrons.